A Study on Simulated Urine Absorption Behaviour of Kapok-Cotton Blended Nonwoven Web

ABSTRACT Textile waste materials such as chemical-based diapers are responsible for aggravating environmental pollution. It usually takes years to degrade naturally. The world is in constant need of some green alternative solutions without compromising on product performance. Cellulosic materials being highly absorbent and biodegradable in nature such as cotton, Kapok, jute, banana etc. have the potential to replace these synthetic materials. The present study investigates the simulated urine absorption and retention characteristics of 100% cotton and Kapok-cotton blended webs. The morphological characteristics of raw, scoured and rewetted Kapok fiber were first studied. The hollow Kapok fiber lumen was found to have collapsed after scouring. However, it regained its shape after re-wetting in simulated urine. Absorption capacity was found to be maximum at 50:50 Kapok-cotton ratio in the nonwoven web. Statistically, the t-test also corroborates the same result at a 5% significance level. The absorption rate (g/s) and retention (%) were also maximum for the same blend. With an increase in external pressure, the liquid retention was reduced. On comparing a 50:50 Kapok-cotton blended with a 100% cotton nonwoven fibrous web, the absorption capacity, rate, and retention are increased by 26.1%, 300%, and 13.5%, respectively.


Introduction
In recent years, environmental pollution due to textile waste has become one of the most alarming concerns worldwide.Textile waste includes wastewater, discarded textile goods, and landfill waste materials such as used diapers, clothing, etc.One of the reports suggested by Khoo et al. (2019) states that diapers being the third largest consumer product have contributed to 30% of non-biodegradable landfill waste.Around 3.5 million tonnes of used diapers end up in landfills annually and require more than 500 years to completely decompose.On the other hand, the production of diapers keeps on increasing annually, at an exponential rate.After disposal in the landfill, heavy metals, dioxins, phthalates, sodium polyacrylate (SPA), and other toxic substance leaches into the soil and water, which can cause bioaccumulation and biomagnification.Consequently, it can lead to severe infectious diseases (Mistry et al. 2023).The world is in immediate need of green alternative and ecological solutions to replace or, at some level limit the consumption of these waste products.The use of cellulosic materials is highly recommended as they are eco-friendly and easily disposable.Though cotton is one of the most used cellulosic materials in textile industries, the pricing increases daily.The liquid absorption and retention capacities of cotton material are also not too high.So, industries are looking for an alternative with similar properties and more cost-friendly.Kapok fiber can be used for the same.Moreover, it is abundantly available, biodegradable, and nontoxic in nature.
Kapok fibers have already been used as fiberfill in pillows, quilts, soft toys, and are applicable in buoyancy, acoustical and support materials, thermal insulation, oil absorbing materials, paper production, biofuel etc (Baraniak and Kania-Dobrowolska 2023).The fundamental nature of Kapok fiber is hydrophobic (due to the waxy layer), oleophilic, and biodegradable (Wang and Wang 2013).The density of its fiber wall is 1.31 g/cm 3 (Karan et al. 2011).The Kapok fiber exhibits a microtube structure with a large hollow lumen (77% fiber volume).The fiber structure is composed of lignin, cellulose, polysaccharide, etc (Cao et al. 2017).The air entrapped within the fiber lumen prevents liquid entry within the fiber, as it has a higher surface tension than Kapok (Hori et al. 2000;Lim and Huang 2007).
Many researchers have demonstrated the influence of alkali treatment on raw Kapok fiber and observed improved hydrophilicity.This treatment removes wax as well as natural oils or pectin from the fiber surface.The wax removal collapses the rigid, hollow structure and partially destroys the lumen (Abdullah, Rahmah, and Man 2010;Liu and Wang 2011).As Kapok fiber has a lower alkali resistance than cotton fiber, using a high alkali concentration could result in a grooved serrated surface.Therefore, relatively mild alkali treatment is recommended during the finishing processes (Liu and Wang 2011).It has been shown that the fiber's hollowness can be restored by treating it with a relatively high concentration (180-280 g/l) of alkali (NaOH) for 60-80s (Hu et al. 2017).However, it is a costly process and can damage the fiber's surface.
In one of the studies (Bozaci 2019), atmospheric plasma, and enzymatic treatment were used to impart hydrophilicity.Sodium chlorite treatment on the fiber imparts hydrophilicity to the fiber (Zheng et al. 2015).Chen et al. (2013) compared the dye absorption of alkaline-treated Kapok, cotton, and C. gigantea fibers and found that the lignin presence in Kapok negatively impacted the dye absorption process.Another researcher reported that Kapok absorbs a meager amount of water than the hollow lumen's actual capacity (Rijavec 2009).
As already stated, the Kapok fiber has been used as an oil absorbent.Numerous researches have already been carried out (Thilagavathi, Karan, and Thenmozhi 2020;Xu et al. 2021).Such as, Liu and Wang (2011) assessed the effect of water, HCl, NaOH, NaClO 2 , and chloroform with varying concentration, temperature, and time on the oil absorbency of Kapok fiber.Singh et al. (2023) have investigated the oil absorbency and retention behavior of raw Kapok-industrial waste cotton blend and found that the 90:10 Kapok-cotton blend shows higher oil absorption (engine oil, vegetable oil, and diesel oil).
The Kapok fiber has a short length and lower strength, so converting 100% Kapok into a nonwoven fibrous web by using the carding machine is impossible.100% Kapok nonwoven (Renuka, Rengasamy, and Das 2016) can be developed by air laying technique.The difficulty in processing 100% Kapok fiber still limits its application.To improve its fabrication possibilities, some researchers have blended it with polypropylene in various ratios (100/0, 75/25, 50/50, 25/75, and 10/90) for nonwoven fabric formation using a needle punching process.The PP/Kapok blend (50/50) sample shows the best oil absorption capacity and lowest bulk density (Lee et al. 2013).The water absorbency (Debnath and Madhusoothanan 2010) and compression (Debnath and Madhusoothanan 2012) behaviors of blended nonwoven are influenced by blend percentage, structural parameters as well and external pressure, under both dry and wet conditions.
Though Kapok is a cellulosic fiber, its potential use in water-absorbing products has not been explored much.Very little literature has been found on the water absorbency of 100% Kapok or in the blended form (Bozaci 2019;Macedo et al. 2020).Hydrophobicity, processing difficulty, and the collapsing tendency of the hollow lumen when subjected to alkaline treatment probably have discouraged the researchers.At the same time, the restoration of the collapsed lumen after alkali treatment has not been attempted by many.Though the capability of Kapok to absorb oil has been researched by many, its capacity to absorb synthetic urine (i.e., saline solution) has not been assessed at all, so that it can be used in diaper products.Hence, it was decided to study the possibility of opening up the collapsed lumen of Kapok fiber after alkaline treatment and its synthetic urine absorption and retention capabilities in nonwoven web form, so that it can exploited in diaper products.

Sample preparation
Kapok [≈20 µm] fibers and 4.7 micronaire cotton fibers were pre-scoured using 4% o/w sodium hydroxide, 2% o/w Lisapol at 100°C for 60 minutes.Now, the samples were thoroughly washed to remove all the traces of alkali.Post-washing, the alkali-treated fibers were opened gently by fingers in a wet state in the form of minute bundles.The fibers were then dried at room temperature for 24 hours.
The Kapok fibers were then blended with cotton in the proportions of 0% (sample A), 50% (sample B), and 70% (sample C). 12 g of the mixture was converted into the parallel-laid web (75 cm × 25 cm) on a miniature carding machine by giving two consecutive passages through the machine.This was done to ensure the homogeneity and individualization of fibers in the web.The carding parameters (feed rate, cylinder speed, licker-in speed, doffer speed, and settings between different organs) were suitably adjusted so that a coherent web could be produced.All the carding parameters were then kept constant.
This was followed by punching of the webs on the needle punching unit of the DILO nonwoven machine.Punching was done from both sides keeping a punch density of 150 punches/cm 2 , for adequate structural integrity.The depth of needle penetration was 10 mm.
Three web samples were produced from each Kapok: cotton mixture.A total of nine web samples were produced.

Thickness
The thickness of all the samples was measured at 0.19, 1.96, 4.90, 9.80, and 19.61 kPa using an Essdiel thickness gauge as per ASTM D1777-96.Care has been taken to avoid the deformation of the samples during handling.Ten readings per test have been taken, and the average was calculated.

Areal density (g/m 2 )
The areal density (mass per unit area) was determined by following ASTM D3776/D3776M.Specimens of 5 cm diameter were cut by a GSM round cutter.The specimens were weighed on an electronic balance.Five readings were taken per sample, and average areal density was determined.The areal density was in the range of 54-63 g/m 2 , 55-65 g/m 2 and 45-60 g/m 2 for samples A, B and C respectively.The average areal density for samples A, B and C were 58 g/m 2 , 62 g/m 2 and 55 g/m 2 , respectively.

Morphological characterization
The morphology of the fiber surface before and after scouring treatment was studied by taking images on a scanning electron microscope (SEM) as well as a Nikon SMZ 1500 optical microscope with reflectance mode.A single fiber was removed and placed over a glass slide with both ends fixed by cello tape.The images were captured by the camera and carefully observed for any change in the morphological character of the fiber.Several fibers were studied in the same way.

Contact angle
Contact angles were estimated with a KRUSS-made Drop Shape Analyzer (AATCC 79-2000) equipped with a particular in-built optical system and camera.A drop of liquid (2μl) was placed on the fabric, and the image was immediately captured by the camera for analysis.At least five observations were made for each sample.

Pore size calculation
The pore size distribution was determined on Porolux 100 based on the gas-liquid displacement principle.A sample (approximately 2.5 cm in diameter) was completely immersed in the profile (surface tension of 16.0 dyne/cm) for 30s.After placing the wet sample in the sample holder of the instrument, the pressure was raised.The instrument provides the bubble point, smallest pore (SP) diameter, mean flow pore (MFP), etc.

Absorption testing
The samples absorption properties were evaluated using a saline solution (simulated urine).As the study focuses on baby diaper study, a saline solution of 9 g of NaCl in 1-liter Deionized water at 23 ± 2°C has been chosen to simulate the urine based on ISO 9073-13 (Bachra et al. 2020;Dhiman and Chattopadhyay 2021).At times, an absorbent product must function under pressure.In diapers, the pressure has been shown to vary from 0.19 to 19.61 kPa (Dey et al. 2016;Dhiman and Chattopadhyay 2021).Therefore, it was thought to study not only the absorption capacity, rate but also retention under pressure.
The GATS test was performed following TAPPI standards T-561.Samples (5 cm diameter) were cut from different locations of the fabric.The specimen was weighed (W) and their thickness (T) were measured following ASTM D1777-96.
The bulk density of each specimen was determined using the following formula: The samples were placed on the platform of the instrument keeping a perforated disk (50 g) on it to generate a pressure of approx.0.19 kPa.The platform holding the sample and the liquid level in the reservoir were brought at the same level.The system delivered liquid to the sample.The instrument generated a graph showing the mass of absorbed liquid by the sample against time and other absorbency-related data.A total six specimens per sample were tested.As the absorption capacity is expressed in g=g ð Þbytheinstrument, the effect of individual specimen weight variations is normalized.

Sink test
The test was performed following the standard NWSP 10.1.The dry weights of 5 cm diameter of samples were measured on an electronic balance (W d ) and after that left within the saline solution for 60 sec.These samples were hung at an angle for 120 sec to drain out the excess liquid.Now, the wet sample weights were determined (W w ).
The absorption capacity (g/g) of the samples was calculated as per the following formula (Chatterjee and Gupta 2002): Six readings were taken per sample and their average absorption capacity was calculated.

Retention test
The fully saturated wet samples (W w Þwere used in this study.First, a deadweight of 500 g was placed on the sample and waited for 240s to allow the liquid to escape.The dead weight was removed, and the sample was weighed ðW f Þ again.After measurement, the sample was taken back, and an additional deadweight of 500 g was placed on top of it for another 240 sec.The weight of the sample was measured again.The process was repeated by adding 500 g till a total weight of 2000 g was reached.The retained liquid is the difference in weight recorded before and after the dead weight was placed.The retention % was calculated for different stages of loading from the formula given below: A total of six tests were performed per sample to find out the average values.

Free swell absorption capacity
The test was performed following the NWSP 240.0.R2 (19) standard.The free swell absorption capacity test refers to the amount (g) of fluid absorbed per gram of the composition (Bachra et al. 2020;Yoon, Chung, and Kim 2003;Zhang, Feng, and Jin 2020).Teabags were water-permeable containers that contained the Kapok/cotton fiber while allowing the liquid to be freely absorbed by the fibers.Rectangular pouch Teabags (20 cm × 10 cm) were formed by using nylon fabric and sealed at corners by seaming.The teabags were filled with 5 g of pre-weighted ðW 1 Þ Kapok/cotton sample, distributed equally through the teabag, and immersed in saline solution for 60 min.The teabags were removed after reaching equilibrium swelling and left for 15 minutes to drain off the excess solution, then weighed (W 2 Þ.Empty teabags were also made and went through the same steps to serve as blanks. The teabag blank's absorption capacity (AC t , g/g) was calculated using the equation.
Here, T 1 andT 2 are the dry and wet weights of the empty tea bags before and after the test.Three samples were tested, and the results were averaged.Similarly, absorption capacity (AC kapok Þof the Kapok fibers can be calculated as follows: Again, three samples were tested, and the results were averaged to find the absorption capacity of the Kapok.Similarly, the absorption capacity for cotton fibers was determined by following the same procedure.

Characteristics of kapok fibre
SEM images (1 µm and 2 µm) of Kapok fiber (cross-sectional and surface) in raw, scoured, and rewet stages are shown in Figure 1.The raw Kapok fiber (Figure 1(i,ii)) has an oval cross-section and is free from convolutions.The hollow lumen is also visible.However, the hollow lumen collapses after scouring and takes a solid shape (Figure 1(iii)).After rewetting the scoured Kapok fiber with Deionized water solution for 24 hours, the collapsed lumen can be found to open partially (Figure 1(v,vi)).(Hu et al. 2017 have also reported a similar phenomenon after treating the Kapok fiber with a relatively high concentration (180-280 g/l) of alkali (NaOH) for 60-80s.
Water was added to raw Kapok fibers on a glass slide, and the wetting process was observed under a microscope at different time intervals.The presence of water and trapped air bubbles within the wet Kapok lumen fiber can be seen in Figure 2. As the water was free to enter from both sides of the fiber, simultaneously, the air may remain trapped between the advancing liquid fronts.Such trapped air can restrict the water absorption within the hollow lumen.

Free-swell absorption capacity
The saline liquid (simulated urine) absorption capacity of Kapok and cotton fibers was determined by the tea-bag method (Figure 3).
The following observation can be made.
• The absorption capacity of cotton samples remains constant over the period (60 min.).The approximate capacity is 21 g/g.• For Kapok fibers, the absorption capacity observed in the first minute is 20.6 g/g.In the 3 rd minute, the capacity rises to 26.5 g/g (≈30%) and after that remains almost constant throughout the test.It, therefore, appears that cotton fibers absorb the saline liquid instantaneously as soon as it is dropped into the liquid.Kapok fibers, on the contrary, absorb slowly at first.Kapok being a hollow fiber, contains air in its hollow lumen.As the bunch of fiber is dropped into the liquid, the liquid rushes in from all directions.Some air may remain trapped within the hollow lumen and be replaced by the liquid slowly.This slight delay in the absorption process was over within three minutes.

Absorption study (GATS and sink testing methods)
The efficacy of the scouring treatment was tested by contact angle measurement (Figure 4) for all webs.
The liquid drop was found to vanish very quickly within the fibrous structure.It indicated the hydrophilic nature of all the webs understudy after the scouring.
The absorption capacity (Figure 5), evaluated by GATS and sink test methods, can be seen to be highest for sample B (50-50% Kapok-cotton) followed by sample C (70-30% Kapok-cotton) and sample A (0-100% Kapok-cotton).A similar observation has been made by Debnath and Madhusoothanan (2010).
Statistical test (t-test at 5% significance level) performed on the data (Tables 1 and 2) shows that a significant difference in the absorption capacities using sink test exists when the 50% Kapok fiber is added in cotton nonwoven webs.However, GATS (Gravimetric Absorbency Testing System) shows no significant difference in the absorption capacities of all three webs.The results are corroborated by Figure 5 for the absorption capacities test by two methods.
The highest absorbent capacity for sample B can be attributed to the lowest bulk density of the web (Table 3).Cotton and Kapok fibers have different cross-sectional shapes (Figure 1).When they are  mixed, the fibers within the web remain poorly packed compared to the packing of fibers in 100% cotton web.As the % of Kapok is increased in the Kapok-cotton blend, the packing of fibers suffers more and more, and the maximum open structure is observed when the percentage of Kapok reaches 50% by weight.It is reflected in the bulk density of the web (Table 3).At 70% Kapok fiber, the quantity of cotton fiber was reduced to 30% only.Kapok being more in number will be able to pack themselves better within the structure, as evident in the increased bulk density value (0.080 g/cm 3 ).Within the lumen of Kapok fibers, some air bubbles can be expected to remain trapped.The trapped air bubble inhibits the penetration of water into the lumen of the Kapok fibers.As Kapok fiber % in the web (>70%) increases, the volume of trapped air within the lumen is expected to increase, which can lead to a reduction in the absorption capacity.

Absorption rate (GATS method)
The absorption rate (Figure 6) can be found to follow an almost similar trend (Figure 5).Sample B shows the highest absorption rate, followed by sample C and sample A. A similar statistical test (t-test at 5% significance level) has been performed on the absorption rate data (Table 4) using GATS.It shows that a significant difference in the absorption rates exists on increasing the Kapok percentage (>50%) in the cotton web.
The absorption rate is directly influenced by the nature of the pores within the web structure, especially the MFP and SP.From Table 3, the mean flow pores (MFP) are practically the same for all three samples.The difference is only observed in the average small pore sizes (SP).It is maximum for sample B followed by samples C and A. The pore size distribution is shown in Figure 7.The inset represents the distribution of smaller size pores (2µm-10µm).The frequencies of the pores close to the average smaller pore size (Inset Figure 7) are maximum for Sample B and practically the same for Sample C and A. Therefore, faster entry of liquid into sample B is expected.

Retention
After completion of the sink tests, the three samples were further evaluated for fluid retention capability.The results are depicted in Figure 8 show:    • The liquid retention % reduces with an increase in external pressure for all the samples (Figure 8).The retention drops sharply initially and then levels off.The overall changes in retention are 89% to 68% for Sample B, 83% to 68% for Sample C, and 78% to 55% for Sample A.
• Sample B shows higher liquid retention% at all pressure levels, followed by sample C and sample A.
As external pressure is applied to the saturated samples, the web thickness reduces, and the liquid moves out from the larger pores since they offer minimum resistance to the liquid flow.At a given external pressure, the fibers and the hydrostatic pressure of the liquid left in the capillaries balance the external load.Bigger pores are transformed into smaller ones and hold the liquid still left (Tavangarrad et al. 2019).
The quantity of liquid expelled due to the liquid retained primarily depends upon the deformability of the web and the liquid already trapped within the pores of the saturated web.
With the increase in external pressure, the web thickness reduces as structural compaction takes place.However, the reduction in thickness becomes less and less with increased compaction of the structure.Therefore, the quantity of expelled liquid from the web is also reduced.It goes on to a pressure level when no more expulsion is possible by external pressure.The liquid is left in fine pores, and capillaries need substantial external pressure.They can only be removed by evaporation.
100% cotton web has inter-fiber pores, whereas Kapok mixed webs have inter-fiber as well as intrafiber pores.The Kapok fibers (samples B and C) liquid remains within the micro-capillary channels, which cannot easily escape as the channels are too fine.Thus, retention in B and C web structures is more than web structure A.

Conclusion
The morphological characteristics of raw, scoured and rewetted Kapok fibers were first studied.The scouring treatment removed the oils and waxes from the kapok fiber surface and appeared to make the surface a little rougher.Kapok fiber lumen was found to collapse after scouring.However, the lumens regained their shape after rewetting.The scoured Kapok fibers have more saline liquid absorption capacity than that of 100% cotton fibers.
Both GATS and sink testing methods showed maximum absorption capacity for 50:50 Kapokcotton nonwoven web compared to 100% cotton and 70:30 Kapok-cotton web.The absorption rate (g/ s) and retention (%) were also maximum for the same blend ratio.With an increase in external pressure, the liquid retention is reduced continuously.Concerning 100% cotton web, the simulated urine absorption capacity and retention of 50:50 Kapok -cotton web improved by 26.1% and 13.5%, respectively.However, the absorption rate increased phenomenally by 300%.

Figure 2 .
Figure 2. Optical microscope observation of water movement within hollow kapok fiber.

Figure 8 .
Figure 8. Retention test of Kapok-cotton nonwoven web at varying external pressure by sink test.

Table 3 .
Physical characteristics of Kapok-cotton blended webs.